This invention relates to optical transmission systems and, more particularly, to devices which perform wavelength conversion of optical signals from a first wavelength to a second wavelength.
Optical devices such as wavelength shifters and wavelength converters have been used to xe2x80x9cchange the wavelengthxe2x80x9d of an optical data signal in optical transmission systems. This change in the wavelength is more properly exemplified as a transfer of data or information from a carrier signal at a first wavelength to a different carrier signal at a second wavelength. Wavelength shifters or converters are integrated into lightwave transmission networks as such networks employ wavelength division multiplexing and wavelength routing of optical signals. The shifters or converters help to overcome the capacity limitations of such networks by rearranging (wavelength channel interchange) and reallocating the optical wavelength channels for efficient use of the limited optical bandwidth of the network.
Optical wavelength conversion has been demonstrated in the past by using a traveling wave semiconductor optical amplifier (SOA) which performs the conversion on intensity modulated optical signals by employing either four-wave mixing or the gain saturation effect. These devices suffer from common shortcomings. The signal extinction ratio (a ratio of the high level of a signal in the optical transmission system divided by the low level of said signal) on the converted signal and/or the conversion efficiency are neither very high nor attractive over a wide range of wavelengths without the expenditure of relatively high input signal power.
Semiconductor optical amplifiers have been used more recently in both Mach-Zehnder and Michelson interferometers to provide wavelength conversion. Amplifiers were arranged in both legs of each interferometer. These configurations exploit the phase shift caused by the refractive index variation associated with gain saturation in the optical amplifiers. As such, the interferometers transfer attendant phase modulation into an amplitude (intensity) modulated signal. This technique apparently operates at low power while improving the signal quality of the converted signal with respect to extinction ratio and chirp. However, both techniques employ SOAs which require the conversion of the optical signal into an electrical signal. Such conversion results in signal degradation or loss that is inherent in the physical devices that perform the conversion. The conversion devices also add cost, complexity and electrical delays in the systems in which they are integrated.
These and other deficiencies of the prior art are overcome according to the principles of the invention in an apparatus for wavelength conversion that includes a first optical beam splitter, a first leg and a second leg each splitting from the first optical beam splitter, a first optically passive nonlinear device disposed in the first leg, a second optically passive nonlinear device disposed in the second leg and a second optical beam splitter joining the first leg and second leg beyond the first and second optically passive nonlinear devices. The first and second optically passive nonlinear devices are slabs of nonlinear optical media and in one embodiment of the invention they are Kerr media.
The apparatus additionally has a first mirror in the first leg and a second mirror in the second leg. The first mirror is disposed between the first beam splitter and the first optically passive nonlinear device; the second mirror is disposed between the second optically passive nonlinear device and the second beam splitter.
The apparatus also has an optical source connected to the first beam splitter. The optical source generates an unmodulated optical signal. A condition is established in the apparatus wherein the unmodulated optical signal is split between the first leg and the second leg and where no other beam is in the second leg, all optical signals exit the second beam splitter at a first port. A second condition is established wherein the unmodulated optical signal is split between the first and second leg and where another beam is imparted to the optically passive nonlinear device in the second leg, an imbalance is created at the second beam splitter such that some portion of the unmodulated optical signal exits at a second port of the second beam splitter in a modulated manner.